This invention relates generally to a low-pressure mercury vapor discharge lamp having a particular type phosphor coating to emit white light when excited by the ultraviolet radiation generated from the mercury vapor discharge. More particularly, the present type lamp construction is intended for general illumination at a color temperature in the range 2,700.degree. -6,500.degree. K with a satisfactory color rendition and at higher emission efficiency than conventional deluxe-type fluorescent lamps.
The use of several luminescent materials in combination to produce a predetermined overall spectral energy distribution is well-known. It is also well-known to employ such material combinations as a blended mixture or as a plurality of two or more layers in which one layer generally further comprises a blend of the individual phosphors. Particularly well-known phosphor blends in the form of a single layer phosphor coating to produce white color emission include two component mixtures for deluxe cool-white and deluxe warm-white. These already well-known two component phosphor mixtures generally employ a manganese-activated or antimony-activated strontium haloapatite phosphor constituent including various combinations thereof. Conventional deluxe-type fluorescent lamps represent a compromise between luminous efficiency (lumen output per watt input) and color rendition since better color rendition is usually achieved with a reduction of as much as 35 percent or more in luminous efficiency. A satisfactory color rendition for deluxe-type fluorescent lamps is approximately 80 or more as measured by the generally accepted C.I.E. color rendering index. The color temperature of the emission in these lamps is also fixed at around 3,000.degree. K for the warm-white deluxe lamp, around 3,500.degree. K for the standard white deluxe lamp, around 4,200.degree. K for the cool-white deluxe lamp, and around 6,500.degree. K for the daylight deluxe lamp, as measured by the C.I.E. chromaticity x and y values.
A relatively recent fluorescent lamp development employs various rare earth oxide phosphors exhibiting higher luminous efficiency than the conventional phosphors to increase the overall efficiency of the phosphor combination. A rare earth oxide phosphor being employed in this manner is generally blended with two or more different phosphor materials to provide efficient composite emission. In a different known embodiment, the relatively expensive rare earth oxide phosphor material is employed as a separate top layer overlying a phosphor blend layer of less expensive phosphors to increase the absorption of ultraviolet radiation by the expensive material and thereby increase the composite emission. Such a combination permits accomplishment of a predetermined spectral energy distribution utilizing less rare earth oxide phosphor material than would be the case if a single phosphor layer of the blended mixture were employed.